Horizontal Liver Cleft: A Rare Anatomic Variant

Background

Improvements in hepatobiliary surgical techniques, with increased usage of segmental and subsegmental resection, make accurate preoperative radiological assessment delineation of the liver segments ever more crucial. Conventionally, this is done by drawing imaginary straight planes along the portal and hepatic veins, but this is riddled with substantial reported inaccuracies.

In this report, we present a case of an incidental cleft between the superior and inferior liver segments. This patient with an unusual liver cleft illustrates the difficulties in liver segmentation using Couinaud’s classification, as discussed below. Its not-so-straight configuration may be a case in point against using straight planes in liver segmentation.

Case Report

We report a 74-year-old female ovarian cancer patient of with peritoneal metastasis and retroperitoneal lymphadenopathy.

On contrast-enhanced CT, apart from the ovarian cancer, we noted the liver had a smooth, well-defined, horizontally orientated cleft that broadly divided the organ into 2 halves (Figure 1). The cleft contained the right and left main portal veins, and consequently had a curved down-sloping configuration accommodating the curved course of these veins. Within the cleft, each portal vein gave rise to respective sectoral branches (Figures 2 and 3). The later in turn gave rise to respective segmental branches that were seen penetrating the liver segments. This liver cleft was present from an earlier CT study performed 3 years ago, and there was no history of preceding liver surgery.

Discussion

Hepatobiliary surgery is an evolving field, and with the increased application of segmental and subsegmental liver re-section for malignant conditions there has been an increase in the demand for accurate radiological segmentation of the liver. Each segment is a self-contained unit of the liver, with centrally positioned branches of the portal vein, hepatic artery, and bile duct, and vascular outflow through the hepatic veins in its periphery [1].

Historically, there have been a number of descriptions of hepatic segmentation, including those by Couinaud, Healy, and Schroy, Goldsmith and Woodburne, and Bismuth. [2–4]. Couinaud’s classification is most commonly used, and it employs imaginary straight planes, the portal and hepatic scissurae, to define the segments of the liver. The portal scissurae contain the vertically orientated hepatic veins, and the hepatic scissurae contain the portal pedicles. The hepatic scissurae divides the liver into cranial and caudal divisions. As this classification uses arbitrary planes to divide the liver into different segments, there are several factors that cause inaccuracies in the radiological determination of the liver segments.

Other authors have also shown discrepancies between the radiological segmentation and intra-operative findings. Nelson et al. [5] showed that CT localization disagreed with the extent observed at surgical resection in 11 of 36 (31%) lesions, and Soyer et al. [6] reported discrepancy in 8 of 36 (22%) cases. There are many areas of discrepancies in liver segmentation, most concerning the right lobe of the liver [7]. In particular, there is difficulty differentiating between segment 6 and segment 7. The assumption that the hepatic scissurae is linear and horizontal causes this difficulty.

The liver cleft seen in this patient appeared to coincide with the “perfusion boundary” between the cranial and caudal segments in each liver sector as defined by the segmental portal vein blood supply (Figure 4). This strengthens the long-held notion that this boundary is curved and down-sloping in configuration and not horizontal and straight, as radiologists assumed. This finding lends further support to authors who advocate the use of 3D reconstructions of the vascular anatomy to accurately delineate the different hepatic segments [8].

The liver has embryological origins from a ventral foregut diverticulum, and its development involves complex interactions between the mesoderm and the epithelial cells of the endoderm [9,10]. The liver is formed around vascular and biliary pedicles forming the lobules, sub-segments, segments, and lobes. The evolution is centrifugal and stops when it encounters the other segments. Early cessation isolates segments and gives the liver a lobular appearance. This change primarily affects the lateral connections of anterior segments but it can also affect the upper connections and it explains a possible separation of a segment from the rest of the liver. This understanding of liver embryology may explain the horizontal cleft reported in this case.

Conclusions

We report a patient with a rare anomaly of a liver cleft that followed the planes of the first branches of the portal vein. This illustrates the difficulty of differentiating the superior and inferior hepatic segments, particularly in the right lobe of the liver. To the best of our knowledge, this is the first report of this anomaly, which may be attributed to early cessation of the embryological formation of the liver.